GEO113 Flashcards
Evidence Of Plate Tectonics: Bathymetry
The study of ocean depths, via line soundings, revealed a mid ocean mountain belt during survers for the trans-Atlantic telegraph cable, in 18050s. Harry Hess proposed ‘seafloor spreading’ in 1962 where magma rises from Earth’s Interior at ridges, creating new seafloor. This explains why sediment is thinner than expected for oceans.
Evidence of Plate Tectonics: Magnetism
Crystals in solidified magma (particularly iron rich minerals) polarises based on the magnetism of Earth, and we can see that in layers of rock in volcanoes or in the sea floor that the orientation has shifted. Using this we can build a chronology of seafloor spreading by tracking the changes in magnetic striping through hthe Potassium-Argon dating method. The determined rate of plate movement on average is 50 mm/year = nail growth speed.
Evidence of Plate Tectonics: Deep Sea Drilling
Development of ships for oil exploration with kilometres of drill pipe on board and the ability to stay still. Vessels have been used to collect rock samples to directly confirm magnetic dating.
Evidence of Plate Tectonics: Earthquake Distribution
Since the 20’s scientists noted earthquakes located at what we now call ocean ridges and trenches, which is evidence for motion. In 1954 Kiwoo, Wadati and Benioff noticed 100s of km long seismic zones dipping at 40 - 60 degrees, parallel to trenches - where ocean floor is sinking. Knowledge increased greatly in the 60’s with intense seismic surveillance to monitor nuclear explosions during cold war.
Evidence of Plate Tectonics: Age of Island Chains
Canadian Geologist Tuzo Wilson, the ‘father of plate tectonics’ notices that Hawaiian islands get older moving north west, in ‘63. Explanation: fixed rising plume of mantle material acts like a blowtorch on an overhead plate, and the plate’s motion creates a chain of islands.
More Recent Plate Tectonic Evidence
GPS: Detailed mapping of current plate motions; data is remarkably similar to those from magnetic analysis, so motions must be fairly constant.
Black Smokers: Gas release expected due to magma eruption at mid ocean ridges.
Lithosphere
Rigid outermost 100km of plate, on average, continental is thicker than oceanic. Split into 20 rigid plates.
Divergent Boundaries:
Where plates separate, magma rises resulting in rifting, volcanism and earthquakes. E.g. Mid-Atlantic ridge (Oceanic), and East African rift Valley (Continental).
These boundaries represent the first stages of ocean formation; extension of crust results in magma rising, resulting in further fracturing, volcanoes and ultimately rifting. The red sea is an example of flooding by ocean and creation of an ocean floor.
Convergent Boundaries: Ocean - Ocean and Ocean - Continent
Oceanic crust is always sub ducted as it is denser than continental, forming trenches which results in Earthquakes, e.g. 2004 Indian Ocean Sunda Arc earthquake and tsunami. Volcanism can also arise at these boundaries, forming island arcs at ocean - ocean boundaries, and mountain chains at ocean - continent boundaries.
Convergent Boundaries: Continent - Continent
Continental crust is too buoyant to be sub ducted, so there is earthquake activity and the thickness of continental crust doubles, forming the highest mountains on Earth such as the Himalayas and Tibetan Plateau.
Transform Boundaries: Continent - Continent
Here plates slide past one another, so lithosphere is neither created or destroyed, hence earthquakes but no volcanoes.
Reconstructing the Past
Plate migrations are studies by joining up ocean isochron lines from opposite sides of divergent boundaires. this has allowed the reconstruction of Pangaea, which broke up 230 m.y.a. Pangaea’s assembly is studied via:
- Magnetic alignment of ancient rocks.
- Rock types and fossils revealing ancient climate from which latitude can be inferred.
- Ancient mountains, e.g. Urals, Appalachians reveal collisions of ancient paleo continents.
Plate Tectonic Mechanism
Motion is driven primarily by the dense cold subducted slab pulling plate, but also by elevation of rising magma under ridge causing uplift and ‘landslide’ from bulges. These are known as ‘slab pull’ and ‘ridge push’. The system acts as a convection cell.
Rock
A naturally occurring solid aggregate of minerals.
Mineral
A solid crystalline element or compound that has been formed as a result of geological processes, consisting of a lattice formed by atomic bonds.
Rock Cycle
The switching between igneous, sedimentary and metamorphic rocks.
- Subduction causes melting, forming igneous rocks.
- Tectonic activity induced mountain formation (orogeny) through folding and crumpling of crust; uplift of igneous and other rocks.
- Uplifted mountains with precipitation leads to weathering of igneous rock, and carried by erosion is then lithified to generate sedimentary rocks.
- Plate collision can metamorphose sedimentary rocks under heat or pressure.
Extrusive Rocks
Formed by magma rapidly cooling on the surface.
Intrusive Rocks
Formed by magma slowly cooling inside Earth’s crust.
Igneous Rocks Characterisation
Characterised according to mineralogy, essentially to how much SiO4 is present, igneous rocks with lots of silicate are felsic, and those with little are mafic.
Igneous Rock Formation: Mid Ocean Ridges
Here convection causes rising mafic mantle material, reduced pressure whilst rising enables it to melt to form magma. Forms mafic rocks such as gabbro and Basalt, which are ultimately covered by a sediment layer.
Igneous rock Formation: Subduction Zones
Water driven off sub ducted slabs changes the composition of overlying mantle, reducing its melting point. Overlying melted rock is a mixture of mafic mantle and felsic crust so intermediate felsic magmas are generated, which are then intruded / extruded to form Diorites / Andesites or Granites/ Rhyolites.
Igneous Rock Formation: Mantle Plumes
In addition to mantle rising to the surface through plate tectonic associated convection cells, material too rises through fixed mantle plumes causing melting of mantle to erupt mafic rocks via hot spot volcanoes, e.g. Hawaii.
